Golf balls lately are predominantly either two-piece solid golf balls or three-piece solid golf balls. These golf balls generally have a structure in which a cover of one layer or a plurality of layers made of various types of resin materials encases a core made of a rubber composition. The core accounts for most of the golf ball volume and exerts a large influence on ball properties such as rebound, feel and durability. Recently, a number of disclosures have been made in which the cross-sectional hardness of the core is suitably adjusted so as to impart a specific core hardness gradient, thereby optimizing the spin properties of the ball on full shots with a driver or an iron and thus enabling the ball to travel an increased distance.
Methods for adjusting the cross-sectional hardness of the core include, for example, suitably adjusting the compounding ingredients in the core rubber composition or the vulcanization temperature and time. With regard to the compounding ingredients in the core rubber composition, it is possible to select the types of co-crosslinking agent and organic peroxide used and to adjust their contents. In terms of co-crosslinking agents, the use of methacrylic acid, acrylic acid and metal salts thereof is known in the field of golf balls.
For example, JP-A S59-75932, JP-A S60-60866 and JP-A H03-207709 describe the joint use of given amounts of zinc acrylate and zinc methacrylate in the base rubber. Also, JP-A 2015-177511 (and corresponding U.S. Pat. No. 7,342,073) disclose art in which a co-crosslinking agent and zinc methacrylate are used together with a halogenated organosulfur compound.
However, although the golf balls in these prior-art publications do relate to art in which zinc acrylate and zinc methacrylate are jointly compounded, the principal object is to enhance durability; such art is unable to provide satisfactory spin properties. Moreover, because fatty acid metal salts are not included in the composition, a hardness profile capable of conferring satisfactory spin properties is not obtained. In addition, there is a concern over chemicals adhering to mixing equipment such as a kneader or roll mill, lowering productivity.